Try playing it back on a Mac, iMac or PC and see if you still have the choppy playback. Use VLC player.
I get terribly choppy 4K playback with 4K 60FPS from P4 Pro, even in Final Cut, (export is fine), but I still will preview with VLC player as it always seems to work.
The video on the iPhone should be from DJIGO4 cache and I don't believe it will have full rez or good playback if in 4K.